Joints are maintained in a secure position in the body via tension provided by soft tissues, such as muscles, tendons, and ligaments. For example, one soft tissue that secures the hip joint is the abductor muscle. The abductor muscle attaches to the pelvic bone at one end and to the femur on the other end and is tensed. In the event of surgery affecting the joint, soft tissue tension is often affected; however, optimal soft tissue tension is not always returned to patient.
Total hip replacement or arthroplasty (“THA”) operations have become increasingly common in the United States, with more than 200,000 such operations occurring annually. A THA procedure can replace portions of the femur and the acetabulum with implants. More particularly, during a THA operation, the femoral head can be removed. The removal of the femoral head allows the two points of attachment of the abductor muscle to come together and results in a loss of tension in the abductor muscle. When a surgeon implants a femoral prosthetic, the two points of attachment are moved and result in the return of soft tissue tension. The size and the arrangement of the prosthetic can determine the amount of soft tissue tension returned to the joint. Especially, osteoarthritis secondary to developmental dysplasia of the hip can present numerous technical challenges to a THA procedure, including altered femoral anatomy, limb length discrepancy, and soft tissue contractures.
After the procedure, problems can arise with implants, including aseptic loosening, polyethylene wear, and dislocation. Dislocation after THA can cause discomfort, inconvenience and expense. The incidence of dislocation is related to many factors; the orientation of the hip replacement implants, implant geometry, patient related factors, and improper soft tissue tensioning, or even soft tissue laxity. Despite much research regarding dislocation after THA, the incidence of dislocation has not significantly declined.
Despite the occurrence of post-surgical dislocation, or even post-surgical discomfort, doctors have routinely relied upon measurements obtained by implanting devices within the deceased. For instance, instruments and devices have been constructed and arranged to be used in deceased test subjects for measuring forces at the hip and/or the implant. Still further, it is not uncommon for the particular body components to be removed and isolated for the application of static loads.
Devices created for such testing do not consider or take into effect the combined stresses experienced by the hip components during life. For instance, a typical testing situation may apply a load at only one particular point of an implant and only at one particular angle. The devices used in such research are not capable of measuring forces from a variety of different load applications, as truly experienced by the hip of an ambulatory person or even the soft tissue tension through a complete range of motion. Importantly, such devices, and the testing situations in which they are arranged, are not designed to take into account the effects and properties of live soft tissues, which have significantly different elastic properties in comparison to dead soft tissues. Finally, these devices and the artificial testing situations in which they are used to develop models, cannot possibly take into account the unique anatomical considerations presented by the individual patient.
Further, intraoperative procedures for measuring forces experienced by the implant and at the hip are non-existent because devices for measuring such forces do not exist. Instead of quantitatively determining the forces experienced by a hip implant that is implanted in the actual patient, a doctor may perform a series of manual tests while observing the hip. The doctor may then use subjective judgment to determine what the doctor believes to be the best arrangement.
Accordingly, devices, systems, and methods designed for obtaining quantitative intraoperative measurements are needed for determining the proper placement and arrangement of an implant specific to the unique anatomy of each particular patient. The devices, systems, and methods will provide quantitative intraoperative measurements that allow the surgeon to select implant size, placement and arrangement that will result in an optimal tension provided by the soft tissues.